powerpc: Provide initial documentation for PAPR hcalls

This doc patch provides an initial description of the hcall op-codes
that are used by Linux kernel running as a guest (LPAR) on top of
PowerVM or any other sPAPR compliant hyper-visor (e.g qemu).

Apart from documenting the hcalls the doc-patch also provides a
rudimentary overview of how hcall ABI, how they are issued with the
Linux kernel and how information/control flows between the guest and
hypervisor.
Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Reviewed-by: Laurent Dufour <ldufour@linux.ibm.com>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Add SPDX tag, add it to index.rst]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190828082729.16695-1-vaibhav@linux.ibm.com

Documentation/powerpc/index.rst

@@ -22,6 +22,7 @@ powerpc
     isa-versions
     kaslr-booke32
     mpc52xx
+    papr_hcalls
     pci_iov_resource_on_powernv
     pmu-ebb
     ptrace

arch/powerpc/kernel/exceptions-64s.S

@@ -1408,22 +1408,9 @@ EXC_VIRT_NONE(0x4b00, 0x100)
  *
  * Call convention:
  *
- * syscall register convention is in Documentation/powerpc/syscall64-abi.rst
- *
- * For hypercalls, the register convention is as follows:
- * r0 volatile
- * r1-2 nonvolatile
- * r3 volatile parameter and return value for status
- * r4-r10 volatile input and output value
- * r11 volatile hypercall number and output value
- * r12 volatile input and output value
- * r13-r31 nonvolatile
- * LR nonvolatile
- * CTR volatile
- * XER volatile
- * CR0-1 CR5-7 volatile
- * CR2-4 nonvolatile
- * Other registers nonvolatile
+ * syscall and hypercalls register conventions are documented in
+ * Documentation/powerpc/syscall64-abi.rst and
+ * Documentation/powerpc/papr_hcalls.rst respectively.
  *
  * The intersection of volatile registers that don't contain possible
  * inputs is: cr0, xer, ctr. We may use these as scratch regs upon entry